LOUISVILLE, Ky. – It’s hard to get about 150,000 people to go from a full-throated roar to library-like quiet. But for a smattering of those who perhaps play blackjack and like the number 21, or, especially, those connected with a heretofore unknown colt named Rich Strike, Churchill Downs, back with a full house after two years of racing under restrictions, fell into mostly stunned silence following the 148th Kentucky Derby on Saturday. A return to normal? Not on the racetrack.